Quote:
Originally Posted by SkyViking
You mentioned using a DIMM to measure seeing, but as I understand it this is done at high frequency and therefore not applicable to deep sky imaging? It seems more akin to planetary imaging where the details that can be teased out with lucky imaging are of course much finer.
For deep sky purposes I think the only reliable measurement is actual FWHM of stars in subframes with integration times of tens of seconds or more.
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Hi Rolf. The DIMM certainly does measure image motion at high speed, but only to get a full bandwidth measure of the effect of the turbulence. The long term seeing FWHM applicable to DSO imaging is then obtained by plugging the DIMM output into the standard seeing model - nicely summarised in the reference.
Measured image FWHM is a fair estimate of seeing, but it also includes mount tracking and guide wobble effects that are automatically removed by a DIMM - these effects are convolved with the atmospheric seeing when imaging, so image FWHM is worse than that from seeing alone. DIMM seems to be the gold standard at present.
ref:
http://www.ctio.noao.edu/~atokovin/papers/pasp2002.pdf